The present invention relates to a process for purifying a gas stream containing krypton (Kr) and/or xenon (Xe), which gas stream contains, in addition, fluorine- and/or fluorosulphur-containing impurities and possibly other impurities that have to be separated from the said gas stream, such as oxygen and/or hydrocarbons (CnHm), the said gas stream then being able to be separated by cryogenic distillation, in order to produce high-purity krypton or xenon, or else possibly being a waste gas from such a cryogenic distillation
The production of xenon or krypton is usually carried using atmospheric air, which air is separated by cryogenic distillation, so as to obtain, on the one hand, the conventionally recovered compounds, namely nitrogen, oxygen and/or argon, and, on the other hand, a gas mixture essentially containing xenon, krypton and impurities chosen from hydrocarbons (CnHm), oxygen and fluorocompounds or fluorosulphur compounds, such as CF4, C2F6 or SF6 for example.
Next, this gas mixture, essentially containing xenon, krypton and impurities, is:
either immediately separated and purified in order to obtain xenon on the one hand and krypton on the other hand;
or put into containers, such as gas bottles, and stored, before being separated and purified, as in the first case, later on.
At the present time, there are several known techniques which can be used to remove the fluorine- or fluorosulphur-containing impurities, such as the compounds CF4, C2F6 or SF6, which may be contained in a gas or a gas mixture.
Among these known techniques, mention may be made of chemisorption, the plasma technique and hot catalytic destruction.
Thus, the technique of chemisorption of the fluorine-containing impurities on an adsorbent material of the phyllosilicate type, as described in document EP-A-863,375, is difficult to use on an industrial scale because of the complexity of the process and raises certain reliability problems.
Other documents propose to eliminate the fluorocompounds, often called PFC (standing for PerFluoroCompounds), by means of polymer membranes.
Thus, document EP-A-754,487 discloses a very effective process for removing by permeation, by means of one or more polymer membranes, the PFCs contained in a gas stream consisting of a carrier gas, such as air, oxygen, nitrogen, helium, CO2, xenon, CO, water vapour, hydrogen, krypton, neon or argon, contaminated with PFCs. This process is particularly suitable for removing the PFCs contained in a gas stream coming from a semiconductor fabrication process.
Moreover, document WO-A-90/15662 describes the use of a permselective membrane for separating a gas mixture, particularly a membrane consisting of an amorphous polymer of the perfluoro-2,2-dimethyl-1,3-dioxol type. The gas mixture may be airxe2x80x94a gas mixture of the nitrogen/oxygen typexe2x80x94which possibly contains one or more organic compounds, such as fluorocarbon compounds or any other volatile compound, such as Freon.
Furthermore, document EP-A-358,915 describes a membrane system for gas separation, in which any degradation of the membrane is minimized or prevented by removing the heavy-hydrogen-type impurities by adsorption of the latter on a bed of active carbon lying upstream of the membrane.
Moreover, documents JP-A-61,187,918 and JP-A-4,322,716 relate to the use of a membrane formed from a fluorocopolymer to remove water vapour from a wet gas, such as HCl, CF4, C2F6 or the like, i.e. to dry such a wet gas.
More generally, mention may also be made of document JP-A-4,016,213 which describes the selective permeation of light alcohols, such as ethanol, contained in a gas mixture through a membrane consisting of a heat-resistant polymer, document JP-A-61,187,918 which teaches the use of a membrane formed from a fluorocopolymer to separate a gas used in a welding operation and document U.S. Pat. No. 5,383,957 which relates to a membrane system that can be used to produce nitrogen from air.
Furthermore, there are other, more or less specific, publications reporting processes which use membranes to separate a gas mixture, namely documents
However, these various techniques are often complex to use on an industrial scale, sometimes generating reliability and safety problems, and requiring, in some cases, considerable investment, and often high operating costs.
In addition, some of these processes cannot be applied to the purification of gas mixtures essentially containing xenon and/or krypton and/or do not allow effective removal of all the impurities likely to be present in the gas stream, namely mainly fluorocompounds or fluorosulphur compounds.
The object of the present invention is therefore to propose an improved process making it possible to achieve effective removal of the fluorine- or fluorosulphur-containing impurities which are contained in a xenon and/or krypton gas stream, which process is easy to employ and of acceptable costs from the industrial standpoint.
In other words, the present invention aims to provide a process for separating and purifying a gas stream containing krypton and xenon, as well as impurities, so as to remove the said impurities (fluorocompounds or fluorosulphur compounds) effectively.
Furthermore, in the case of a krypton/xenon mixture that has to be separated into each of these components, the invention also aims to provide, as an alternative, a process for the effective separation of the krypton/xenon mixture thus obtained and this separation being carried out in order to produce, on the one hand, high-purity xenon and, on the other hand, high-purity krypton, combined with effective removal of the said fluorocompounds or fluorosulphur compounds.
The invention therefore relates to a process for removing at least some of the gaseous fluorocompounds and/or fluorosulphur compounds present in a gas feed stream containing xenon and krypton, in which:
(i) the gas feed stream containing xenon and/or krypton and at least the said gaseous fluorocompounds and/or fluorosulphur compounds are brought into contact with at least one first membrane;
(ii) a production gas containing xenon and/or krypton stripped of at least some of the said gaseous fluorocompounds and/or fluorosulphur compounds is recovered on the output side of at least the said first membrane.
Within the context of the invention, the term xe2x80x9cmembranexe2x80x9d denotes any permeation means, especially membranes taken as they are, but also membrane modules, especially membrane modules based on hollow fibres, or else ceramic or similar membranes.
Moreover, within the context of the invention, the term xe2x80x9coutput sidexe2x80x9d should be understood to mean that side of the membrane from which the gas produced is recovered, that is to say the gas substantially purified of fluorine- or fluorosulphur-containing impurities. In the case of a membrane operating by conventional permeability, the xe2x80x9coutput sidexe2x80x9d is the permeate side, whereas in the case of a membrane operating by reverse permeability, the output side is the retentate side.
By analogy, within the context of the invention the term xe2x80x9cwaste gas sidexe2x80x9d should be understood to mean the opposite side of the membrane to the one where the gas produced is recovered, that is to say the side of the membrane via which a gas enriched with fluorine- or fluorosulphur-containing impurities leaves. In the case of a membrane operating by conventional permeability, the xe2x80x9cwaste gas sidexe2x80x9d is the retentate side, whereas in the case of a membrane operating by reverse permeability, the xe2x80x9cwaste gas sidexe2x80x9d is the permeate side.
According to a first variant, the invention also relates to a process for removing at least some of the gaseous fluorocompounds and/or fluorosulphur compounds present in a feed gas formed from a mixture of xenon and krypton, in which:
(i) the feed gas containing xenon and krypton and the said gaseous fluorocompounds and/or fluorosulphur compounds is brought into contact with at least one first membrane;
(ii) a production gas formed from a mixture of xenon and/or krypton stripped of at least some of the said gaseous fluorocompounds and/or fluorosulphur compounds is recovered on the output side of at least the said first membrane;
(iii) the production gas formed from a mixture of xenon and/or krypton is subjected to at least one cryogenic distillation step;
(iv) after cryogenic distillation, a stream of krypton, substantially free of the said gaseous fluorocompounds and/or fluorosulphur compounds, and/or a stream of xenon, substantially free of the said gaseous fluorocompounds and/or fluorosulphur compounds, is recovered.
According to a second variant, the invention also relates to a process for removing at least some of the gaseous fluorocompounds and/or fluorosulphur compounds present in a recycling gas coming from a process generating at least the said recycling gas, the said recycling gas containing xenon and/or krypton and containing, in addition, gaseous fluorocompounds and/or fluorosulphur compounds as impurities, in which:
(i) the recycling gas containing xenon and/or krypton and the said gaseous fluorocompounds and/or fluorosulphur compounds is brought into contact with at least one first membrane;
(ii) a production gas containing xenon and/or krypton stripped of at least some of the said gaseous fluorocompounds and/or fluorosulphur compounds is recovered on the output side of at least the said first membrane, preferably at least some of the said production gas then being sent to the process which generates at least the said recycling gas in order to be possibly be reused therein.
Depending on the case, the process of the invention may comprise one or more of the following characteristics:
in step (ii), a stream of waste gas containing at least some of the said gaseous fluorocompounds and/or fluorosulphur compounds is recovered on the waste gas side of the said first membrane. This stream of waste gas is substantially at the same pressure as that of the stream of feed gas entering the said first membrane, but it contains a higher proportion of fluorocompounds or fluorosulphur compounds than that of the feed gas, that is to say the stream of waste gas is enriched in fluorocompounds or fluorosulphur compounds compared with the stream of feed gas;
the gas feed stream contains at least 40% xenon and/or krypton, preferably at least 50% xenon and/or krypton, preferably at least 70% xenon and/or krypton, preferably at least 80% xenon and/or krypton, and/or the gas feed stream contains xenon and krypton with a concentration of more than 99.999%;
the gas feed stream contains from 1 ppb to 300,000 ppm of fluorocompounds or fluorosulphur compounds, preferably 10 ppb to 50,000 ppm of fluorocompounds or fluorosulphur compounds;
at least part of the gas feed stream is a stream of a waste gas coming from one or more cryogenic distillation columns, which waste gas has, optionally, been pretreated, put into containers and/or stored;
the fluorocompounds or fluorosulphur compounds to be removed are chosen from the compounds CF4, C2F6, SF6 and mixtures thereof;
the gas feed stream contains from 0.01 ppm to 1000 ppm of CF4, from 0.01 ppm to 1000 ppm of SF6 and/or from 0.01 ppm to less than 200 ppm of C2F6;
the stream of production gas, recovered in step ii) on the output side of the said first membrane and containing at least some of the xenon and/or krypton, is introduced into the feed inlet of at least one second permeation membrane;
the stream of waste gas, recovered in step ii) on the waste gas side of the said first membrane and containing at least some of the said gaseous fluorocompounds and/or fluorosulphur compounds, is introduced into the feed inlet of at least one third permeation membrane;
the gas feed stream is at a temperature ranging from xe2x88x9210xc2x0 C. to +100xc2x0 C., preferably at a temperature ranging from 0xc2x0 C. to +60xc2x0 C. If necessary, the temperature of the gas feed stream may be regulated or adjusted in order to bring it into or maintain it in this temperature range, by heating or cooling, depending on the case;
the gas feed stream is at a pressure ranging from 1 bar to 50 bar, preferably the stream is at a pressure ranging from 3 bar to 30 bar and even more preferably about 6 bar to 12 bar approximately, while it is being brought into contact with the said first permeation membrane. If necessary, the pressure of the gas feed stream may be regulated or adjusted in order to bring it into or maintain it in this pressure range;
at least one membrane is made of a polymer. For example, membranes that can be used within the context of the invention are described in document EP-A-754,487, incorporated here by reference, or in other documents cited by the said document EP-A-754,487;
the gas feed stream contains, in addition, impurities chosen from hydrocarbons (CnHm, such as methane and hydrocarbons in which n=2 or n=3, and at least some of the said impurities are removed by oxidative catalysis of the said impurities into CO2 and H2O, in the presence of oxygen and at a temperature of between 80xc2x0 C. and 600xc2x0 C., preferably about 150xc2x0 C. to 500xc2x0 C.;
at least some of the CO2 and H2O impurities, which are produced by oxidative catalysis or are possibly present in the stream of feed gas, are removed by adsorption or drying on at least one adsorbent, preferably at least one adsorbent being chosen from zeolites, aluminas or silica gels;
the adsorbent comprises a type A zeolite or faujasite, preferably a type X zeolite having an Si/Al ratio of between 1 and 1.10, and/or an activated alumina, preferably an activated alumina containing, or impregnated with, metal cations, particularly potassium or sodium cations. Of course, the zeolite may also be exchanged with metal cations, such as lithium, calcium or similar cations;
the adsorption step is carried out at a temperature of between approximately xe2x88x9240xc2x0 C. and +100xc2x0 C., preferably at a temperature of between +5xc2x0 C. and +50xc2x0 C., and/or at an adsorption pressure of between 1 and 100 bar, preferably between 1.1 bar and 50 bar;
the adsorption step is performed according to a PSA (Pressure Swing Adsorption) or TSA (Temperature Swing Adsorption), preferably TSA, cycle;
the production gas recovered in step ii) is subjected to at least one step of cooling it to a temperature of less than xe2x88x925xc2x0 C., preferably to a temperature ranging from xe2x88x9210xc2x0 C. to xe2x88x92150xc2x0 C.;
the stream of production gas, recovered in step ii) on the output side of the first membrane, is introduced into the feed inlet of a second membrane, a second waste gas is recovered on the waste gas side of the said second membrane and that the said second waste gas is introduced into the gas feed stream;
the stream of waste gas recovered in step ii) on the waste gas side of the first membrane is introduced into the feed inlet of a third membrane, a second production gas is recovered on the output side of the said third membrane and the second production gas is introduced into the gas feed stream.
Furthermore, the invention also relates to a process for removing at least some of the gaseous fluorocompounds or fluorosulphur compounds, particularly CF4, C2F6 and/or SF6, present in a xenon and/or krypton gas stream, preferably a mixture of xenon and krypton, in which at least some of the said fluorocompounds are separated by one or more permeation steps, by means of one or more membranes of the conventional or reverse type, depending on the case.
According to another aspect, the invention relates to a plant for removing the gaseous fluorocompounds and/or fluorosulphur compounds present in a feed gas formed from xenon and/or krypton, capable of carrying out a process according to the invention, which comprises:
at least one source of a feed gas containing xenon and/or krypton and gaseous fluorocompounds and/or fluorosulphur compounds to be removed;
means for compressing the feed gas, such as a gas compressor;
at least one membrane connected via its production output to means for recovering a production gas formed from xenon and/or krypton and stripped of at least some of the said gaseous fluorocompounds and/or fluorosulphur compounds.
As a variant, the invention also relates to a plant for removing the gaseous fluorocompounds and/or fluorosulphur compounds present in a feed gas formed from xenon and/or krypton, capable of carrying out a process according to the invention, which comprises:
at least one source of a feed gas containing xenon and/or krypton and gaseous fluorocompounds and/or fluorosulphur compounds to be removed;
means for compressing the feed gas;
at least one membrane connected via its production output to means for recovering a production gas formed from xenon and/or krypton and stripped of at least some of the said gaseous fluorocompounds and/or fluorosulphur compounds;
means for cooling the production gas down to a cryogenic temperature, the said cooling means being located downstream of at least the said membrane;
means for cryogenic distillation of the production gas, the said distillation means being located downstream of the means for cooling down to a cryogenic temperature;
recovery means, located downstream of the cryogenic distillation means, for recovering krypton substantially free of the said gaseous fluorocompounds and/or fluorosulphur compounds, and/or xenon substantially free of the said gaseous fluorocompounds and/or fluorosulphur compounds.
According to one particular embodiment, the plant of the invention includes, in addition, at least one membrane connected, via its waste gas output, to means for recycling the waste gas, which contains xenon and/or krypton and possibly gaseous fluorocompounds and/or fluorosulphur compounds, the said waste gas recycling means being connected, in addition, to the feed input of the said compression means so as to be able to feed the said compression means with at least some of the said waste gas.
More generally, within the context of the present invention, it has been demonstrated that permeability and selectivity properties of the membranes, particularly polymer membranes, can profitably be used to separate, in a very effective manner, the krypton and/or xenon molecules on the one hand, and the CF4, C2F6 and SF6 impurities on the other hand.
This is because, given that fluorosulphur compounds of the SF6 type on the one hand and fluorocompounds, particularly the compounds CF4 and C2F6, on the other hand permeate only very slightly, or even not at all, through conventional, especially polymer, membranes, gas mixtures containing krypton and xenon and contaminants of the CF4, C2F6 and/or SF6 type can be effectively separated by profiting from the very high selectivity of the membranes, particularly the polymer membranes, for xenon and krypton compared with fluorine- or fluorosulphur-containing impurities.
During the separation, the xenon and krypton come together again and are recovered essentially on the output or permeate side of the membrane or membranes used when these are of the conventional type.
In return for the very great simplicity of the process, it should be recalled that a very high selectivity is required in order to claim to separate the molecules with a high efficiency or with high purity.